WASHINGTON — It seemingly happened in the blink of an eye. All of the Islanders’ good feeling and momentum was gone, poof, as if in a cloud of ice chips.

In reality, Tuesday night collapsed in 15 minutes of game time in the second period, when the Islanders allowed five goals — five quick strikes that dug the hole for their 6-2 loss to the Capitals at Verizon Center.

“It should be embarrassing,” said captain John Tavares, whose team had just garnered some good feeling with consecutive wins over the Senators and Bruins over the weekend. “We come in here, we just don’t play our game. We start off OK, but I don’t know why we just don’t stick with it. We have to challenge ourselves in here, we have to be better.”

The most embarrassing part of the night was on the penalty kill, as the Islanders (6-6-3) managed to take six penalties and allow four power-play goals. Two of them were from the Capitals captain, Alex Ovechkin, making his return to the lineup after missing two games with an upper-body injury, and not disappointing any of the 18,506 in attendance, taking his team to 8-7-0 and extending its win streak to three.

“If we want to give those guys, 8 and 19 [Ovechkin and Nick Backstrom] those easy plays in front of the net, it’s going to make it tough on our goaltender,” said Tavares, whose team gave up five goals in a period for the first time since Nov. 28, 2008, against the Bruins in Boston, according to MSG. “We left him out to dry.”

“Him” would be Evgeni Nabokov, making his 12th start in the first 15 games and who played about as well as one could reasonably imagine for a goalie giving up six goals.

“I think we lost the game in 15 minutes,” said Nabokov, who finished with 33 saves. “I think the consistency is not there, discipline consistency.”

After the first period, the Islanders were leading 1-0 on a power-play goal of their own from Tavares, his seventh of the season. But just over three minutes into the second, John Carlson converted on a bad turnover from Kyle Okposo, and 75 seconds after that, Ovechkin got the first of his tallies, now giving him 12 on the year, just one behind the Blues’ Alex Steen for the league lead.

“I thought we played pretty good up until bang-bang on that power play,” Isles coach Jack Capuano said. “We never recovered from that. You can’t give up four power-play goals. That’s attention to detail and something we have to get better at.”

Capuano’s penalty kill has now given up eight goals on 18 chances in the past five games. They could have come through when Okposo tied the game 2-2, but instead they sandwiched Alex Urbom’s even-strength goal with power-play tallies from Marcus Johansson and Ovechkin to allow the Caps to open up a 5-2 lead.

“Everything really,” is how Matt Martin described what went wrong on the penalty kill. “We’re not going to win any games if our special teams play like that.”

The Isles continue their four-game road trip Thursday in Carolina, and without consistency, they could be coming home rather disappointed.

“There is going to be adversity,” Tavares said. “That has to be there on a constant basis, that work ethic, playing at a high pace and winning battles and competing. Those things you can control, and those things weren’t there in the second half of the game. We have to be better.”

Forward Colin McDonald missed his fifth straight with a left hip injury. He is likely to return Thursday.